The invention relates to a circuit for automatic gain control.
In communication technology it is frequently required to control audio signals, for example speech or music signals so as to maintain them at a substantially constant level. This process is generally referred to as gain control. In particular in the field of entertainment electronics, it is desirable, mainly for reasons of convenience, to have automatic gain control without the intervention of the user. Automatic gain control circuits are employed in practically all video recorders and in many compact-cassette recorders. The basic problem in automatic gain control systems is that on the one hand gaturation of the tape should be avoided and on the other hand the systems should not be too sensitive to individual pulses.
DE-OS No. 30 27 715 describes a circuit for automatically controlling or limiting of the dynamic range in which the emphasis is put on a fast response of the control system. This circuit comprises two inputs through each of which a capacitor is charged. The time constants are selected in such a way that the charging-time constant and the discharge-time constant of the first capacitor are very small and those of the second capacitor are significantly larger. However, in particular in the case of peak rectification of the signal this leads to an oversensitivity to brief high-frequency signal components. This means that in the case of such signal components the control system reduces the audio signal considerably and subsequently increases this signal too slowly, which sounds annoying. This "flutter" effect results in a very unsteady sound impression. Generally, this becomes even worse when a noise-reduction circuit is used.
Further, DE-OS No. 23 01 281 describes an automatic gain control circuit by means of which it is attempted to overcome this problem by the use of two control inputs with different response times and different recovery times. For example, in the case of signal components or noise of short duration the recovery time should be small. However, this leads to the above-mentioned fluttering and unsteady sound impression, in particular if the control circuit is so arranged in a recording amplifier that it receives the pre-emphasized audio signal. However, if it is so arranged in the recording amplifier branch that it receives the signal without pre-emphasis, this results in the treble tones being distorted because the control circuit processes these tones in the same way as the mid-range tones and the bass-tones. No allowance is then made for the fact that the treble tones are pre-emphasized after the gain control.